Ultrasonic welding a critical process for manufacturing face masks. For most masks, a cup-shaped filter media is bonded by ultrasonic welding to a substrate, or shell. The welding is performed around the outer periphery with a desired pattern. Depending on the product requirements and process conditions, several patterns are used, including the waffle pattern, peripheral weld pattern, and eyebrow pattern, shown in FIGS. 1A, 1B, and 1C. In some cases the patterns are used for both cutting and sealing operations. A common feature in all patterns is that the internal diameter is greater than 7-10 cm (3-4 in).
Current horns used for ultrasonically welding face masks generally have an outside diameter of 15 cm (6 in). The horn is formed with multiple slots machined radially through the center, as shown in FIG. 2. The slots are intended to control the amplitude profile of the horn at the weld surface. Without the slots, the amplitude varies greatly on the weld surface. In some cases, the welding amplitude at certain locations is zero and no welding occurs at these locations.
This slotted horn is used to perform peripheral welding of face masks and tacking of the layers. Normally a pattern is machined on the end of the horn opposite the input or excitation end. This design is fairly complicated to machine, and has proven to be unreliable in production. This has resulted in significant cost to fabricate spare horns, and in excess machine downtime. Also, the vibration amplitude on the weld surface varies 25-30% along the weld pattern. In order to achieve acceptable welds with this non-uniformity, the dwell time of the welding process must be increased, which results in reduced throughput of the entire machine. Nonuniformity can result in overwelding at some locations, or underwelding at others. This reduces the process window. For some products which contain thicker layers or increased numbers of layers, higher welding amplitudes are required. In such cases, this horn is limited, because higher amplitudes cause rapid failure. Moreover, the slots increase stresses in the horn and the horn is likely to fail at these slot locations.
Other designs are available, such as the "bell-shaped" horn, with or without slots, as shown in FIG. 3. None of these designs has been found to be totally acceptable due to displacement nonuniformity, amplitude restrictions, or premature failure.